This invention relates generally to Global Orbiting Navigation System (xe2x80x9cGLONASSxe2x80x9d) signal receivers. More particularly, the present invention relates to a novel technique for calibrating GLONASS receivers for use in making survey measurements with sub-centimeter accuracy. GLONASS is a global navigation system developed in the former Soviet Union to perform the same functions as the Global Positioning System (GPS) developed in the United States. Receivers are being developed to process signals from both systems of satellites. Having more satellites available to a receiver results in a faster convergence on an accurate position result and, if visibility is limited by geographic or architectural obstructions, may provide for location determination that would not have been obtainable if only one set of satellites were used.
There are, however, design differences between GPS and GLONASS that have an impact on high accuracy applications. The present invention addresses a significant problem arising from one of these differences, as will be discussed below after first providing some background on GPS.
Overview of GPS:
The global positioning system (GPS) may be used for determining the position of a user with a GPS receiver located on or near the earth, from signals received from multiple orbiting satellites. The orbits of the GPS satellites are arranged in multiple planes, in order that the signals can be received from at least four GPS satellites at any selected point on or near the earth.
The nature of the signals transmitted from GPS satellites is well known from the literature, but will be described briefly by way of background. Each satellite transmits two spread-spectrum signals in the L band, known as L1 and L2, with separate carrier frequencies. Two signals are needed if it is desired to eliminate an error that arises due to the refraction of the transmitted signals by the ionosphere. Each of the carrier signals is modulated in the satellite by at least one of two pseudorandom noise (PRN) codes unique to the satellite, and transmitted as a spread spectrum signal. This allows the L-band signals from a number of satellites to be individually identified and separated in a receiver. Each carrier is also modulated by a slower-varying data signal defining the satellite orbits and other system information. One of the PRN codes is referred to as the C/A (clear/acquisition) code, while the second is known as the P (precise) code.
In the GPS receiver, signals corresponding to the known P code and C/A code may be generated in the same manner as in the satellite. The L1 and L2 signals from a given satellite are demodulated by aligning the phases, i.e., by adjusting the timing, of the locally-generated codes with those modulated onto the signals from that satellite. In order to achieve such phase alignment the locally generated code replicas are correlated with the received signals until the resultant output signal power is maximized. Since the time at which each particular bit of the pseudorandom sequence is transmitted from the satellite is known, the time of receipt of a particular bit can be used as a measure of the transit time or range to the satellite. Because the C/A and P codes are unique to each satellite, a specific satellite may be identified based on the results of the correlations between the received signals and the locally-generated C/A and P code replicas.
Each receiver xe2x80x9cchannelxe2x80x9d within the GPS receiver is used to track the received signal from a particular satellite. A synchronization circuit of each channel provides locally generated code and carrier replicas, which are synchronous with each other. During acquisition of the code phase within a particular channel, the received satellite signal is correlated with a discrimination pattern comprised of some combination of xe2x80x9cearlyxe2x80x9d and xe2x80x9clatexe2x80x9d versions of the channel""s locally generated code replica. The resultant early-minus-late correlation signals are accumulated and processed to provide feedback signals to control code and carrier synchronization.
Although there are several ways to create a spread spectrum signal, the one most often used is xe2x80x9cdirect spreadingxe2x80x9d with a pseudorandom code, which is the technique used in GPS. A direct sequence spread spectrum signal is normally created by biphase modulating a narrowband signal with a pseudorandom code. Each GPS satellite normally transmits three spread spectrum navigation signals. One is on the L2 carrier signal and is based on the P code from a P code generator, and two are on the L1 carrier signal and are based on the P code from the P code generator and the C/A code from a C/A code generator, respectively. To accomplish this, the L1 carrier signal is first divided into two components that are in phase quadrature. Each of these components is individually modulated with navigation signals before being combined, amplified, and transmitted.
The frequency spectrum resulting from this process is one in which the original carrier frequency at F0 is suppressed, and the total signal energy is spread over a bandwidth around F0 of plus and minus the code clock frequency to first nulls. Spectral components outside this bandwidth also are created, but at ever lower amplitude with frequency separation.
The key functions of each satellite are all driven by a single clock with a frequency of 10.23 MHz . The L1 carrier frequency of 1575.42 MHz is obtained by multiplying 10.23 MHz by 154. The L2 carrier frequency of 1227.6 MHz is 120 times the clock. The P code rate is 10.23 MHz and is obtained directly from the clock. The C/A code rate is one tenth the clock frequency and is obtained through a frequency divider. Even a 50 bit per second data rate used to retrieve data from a memory is derived from the same clock. It can be said that all of these signals are coherent because they are derived from a single clock.
In a typical GPS receiver, a single antenna collects all available signals, which are processed through a filter, an amplifier and a downconverter to obtain a lower intermediate frequency (IF) for further processing. Only then is the composite signal digitally sampled, to facilitate further processing of the signals in digital form.
A key aspect of the GPS design is that each satellite uses the same L1 and L2 carrier frequencies but pseudorandom code sequences (P-code and C/A-code sequences) that are unique to the satellite. In other words, the satellites are identified in a receiver by their unique pseudorandom code sequences.
The GLONASS Approach, and the Problem:
Each satellite in GLONASS uses the same pseudorandom code sequences but uses unique L1 and L2 frequencies. Thus, a GLONASS receiver identifies satellites by their carrier frequencies and not by their pseudorandom code sequences. Specifically, each satellite in GLONASS transmits on a frequency in the bands 1,597-1,617 MHz for L1 and 1,240-1,260 MHz for L2. The channel center frequency spacing is fixed at 0.5625 MHz for L1 and 0.4375 MHz for L2.
As originally conceived, both systems were designed to compute receiver locations for navigation purposes, based on measurements made of the arrival times of the pseudorandom code sequences from each satellite in view of the receiver. It was later found that receiver locations could be determined much more accurately by using measurements of the carrier phase. Receivers using carrier phase for location determination form a distinct and increasingly important class of positioning receivers referred to as kinematic processing receivers or survey receivers. These highly accurate receivers find application in survey work, in aircraft landing systems and in earth moving or landscaping machines.
A difficulty arises in processing GLONASS carrier frequency signals simply because they are different for different satellites. Each received carrier signal can be identified and isolated either with a separate, narrow bandpass filter for each satellite channel, or accounted for in digital data processing. In the preferred embodiment the satellite frequencies are selected and separation between channels is achieved in digital data processing. Prior to digitizing, a bandpass filter is used, encompassing all satellite frequencies and establishing the sampling bandwidth for an analog-to-digital (AID) converter. The group delay within the bandpass of this filter will introduce large, unknown delays between satellite channels, which will greatly deteriorate the receiver position measurement accuracy. A similar situation would arise if an individual narrowband filter were used for each satellite channel. It is virtually impossible to design a narrowband filter that introduces the same delay over a range of frequencies. Therefore, the carrier signal from any two GLONASS satellites will be subject to two different phase delays in the bandpass filter used to process the signals in a receiver. Kinematic processing, however, inherently requires the carrier signals received from the satellites to be referenced or compared. In simple terms, the arrival times of the carrier signals from two satellites are compared in kinematic processing. Differences in the carrier frequencies can be accounted for, but differences in phase delay caused by narrow-band filtering cause a very significant problem. Comparison of one carrier signal with another from a different satellite is rendered highly inaccurate because each signal is subject to a different delay in filtering. GPS satellite signals do not have this problem because all the received GPS signals have the same frequency and are filtered in a common filter. Separation into different xe2x80x9cchannelsxe2x80x9d corresponding to different satellites is effected later in processing, when the pseudorandom codes are identified. Thus, any A phase distortion affects the signals from different GPS satellites equally.
A distance of one centimeter is equivalent to a signal propagation delay of approximately 30 picoseconds at the carrier frequencies used in GLONASS and GPS. Thus, for one-centimeter accuracy, either the time delay variation between GLONASS channels must be smaller than 30 picoseconds (secondsxc3x9710xe2x88x9212), or there must be some provision to calibrate the channels to that accuracy. With currently available receiver bandpass filters, group delay variations approaching 30 picoseconds throughout the passband cannot be achieved. Typical low-cost ceramic filters exhibit a delay variation of 5-10 nanoseconds (secondsxc3x9710xe2x88x929). Even the best surface acoustic wave (SAW) filters have at least 400 picoseconds average group delay in the passband, in addition to a delay ripple of much larger amplitude.
The principal group delay variation sources in radio frequency receivers are detection bandwidth determining elements, i.e., the RF (radio frequency) and IF (intermediate frequency) bandpass filters. Group delay variations in the passbands of these filters are due to a combination of: (a) delay-versus-frequency nonlinearities inherent in filter design, (b) production tolerances and (c) sensitivity to temperature changes and aging of components. Conceivably, a GLONASS receiver could be designed, constructed and then calibrated in such a way as to measure the variations (a) and (b) and to compensate for them, because these variations could be expected to remain constant. Slowly changing variations due to temperature changes and aging present a more difficult problem.
It will be appreciated from the foregoing that there is a need for a solution to the difficulties inherent in using GLONASS receivers for position determination to sub-centimeter accuracy. In particular, there is a need for a GLONASS receiver in which carrier phase measurements are not affected by channel-to-channel differences in distortion introduced by bandpass filters, or by slow variations in distortion caused by temperature changes or aging of components. The present invention satisfies this need, as will become clear from the following summary.
The present invention resides in a technique for periodically calibrating each of multiple channels in a GLONASS receiver, to ensure that there are no differences in carrier phase delay from channel to channel. Briefly, and in general terms, the receiver of the invention comprises an antenna subsystem for receiving signals from a plurality of orbiting satellites; a receiver channel coupled to the antenna subsystem, designed to receive and process all signals from a plurality of orbiting satellites, wherein the entire spectrum of satellite signals is passed through a bandpass filter for further processing; and a calibration channel, including a single narrow bandpass filter centered at a selected intermediate frequency. The calibration channel also includes means for processing signals from each of the satellites in turn through the single filter, to provide for each channel a reference carrier phase measurement that is independent of effects arising from the use of multiple bandpass filters in the plurality of conventional receiver channels.
More specifically, the means for processing signals from each of the satellites in turn through the filter includes a first frequency mixer, for downconverting signals received from the antenna subsystem to the selected intermediate frequency; a second frequency mixer, for upconverting signals output by the narrow bandpass fitter, by the same frequency that the signals were downconverted in the first frequency mixer; a local signal generator providing to the first and second frequency mixers a signal at a frequency that is the difference between the carrier frequency of signals received from a selected satellite and the intermediate frequency; and means for applying satellite selection signals to the local signal generator, to effect selection of different satellite signals in turn for processing through the narrow bandpass filter. The receiver may further comprise means for storing the reference carrier phase measurements corresponding to each of multiple receiver channels; and means for computing the difference between a carrier phase measurement from each conventional receiver channel and the reference carrier phase measurement corresponding to the same channel, to obtain a calibrated carrier phase measurement for each channel.
The means for storing the reference carrier phase measurements includes a plurality of phase measurement storage cells; and a demultiplexer having an input derived from the calibration channel, multiple outputs coupled to the phase measurement storage cells, and a control signal input receiving the same satellite selection signals that are used to control the local signal generator.
Further, the calibration channel and the conventional receiver channel may each include an analog-to-digital converter. The carrier phase measurements and reference carrier phase measurements are converted to digital form, for demultiplexing, storing and further processing. Another feature is that the calibration channel further includes first and second wide passband filters to remove unwanted frequency images that are a necessary by-product of frequency mixing.
The invention may also be defined as a method for calibration of a global orbiting satellite system (GLONASS) receiver, comprising the steps of receiving signals from a plurality of orbiting satellites, each distinguished by use of a different carrier frequency band; separating the satellite signals in digital processing, each having a different frequency offset for each satellite; obtaining from the conventional receiver channel a set of carrier phase measurements that is subject to errors resulting from the bandpass filter group delay variation; processing the received signals in a calibration channel that has a single bandpass filter centered at a selected intermediate frequency; and generating in the calibration channel a set of carrier phase reference measurements that can be used to correct the carrier phase measurements from the conventional receiver channel. More specifically, the step of processing the received signals in the calibration channel includes downconverting the received signals by a difference frequency selected to position the frequency band of signals from a selected satellite over the intermediate frequency; bandpass filtering the downconverted signals; upconverting the signals after bandpass filtering, by the same selected difference frequency used in the downconverting step; and periodically selecting a different satellite by changing the selected difference frequency used in the downconverting and upconverting steps. Moreover, step of generating a set of carrier phase reference measurements includes tracking and measuring the carrier phase for each of the selected satellites, and storing the carrier phase measurements resulting from the foregoing processing steps.
Even more specifically, the step of generating a set of carrier phase reference signals further comprises receiving reference carrier phase measurements from the calibration receiver; and distributing the reference carrier phase measurements to separate storage units corresponding to the separate satellites. The method may also include the step of computing corrected carrier phase measurements by computing the difference between the carrier phase measurements obtained from the conventional receiver channels and the corresponding reference carrier phase measurements stored in the storage units.
It will be appreciated from the foregoing that the present invention represents a significant advance in the field of global positioning receivers, particularly receivers for use with GLONASS. Specifically, the invention provides a technique for calibrating a conventional GLONASS receiver to allow it to be used for position measurements at sub-centimeter accuracy. Other aspects and advantages of the invention will become apparent from the following more detailed description, taken in conjunction with the accompanying drawings.